Novel precious metal sensitizing solutions

ABSTRACT

OPTICALLY CLEAR SENSITIZING SOLUTIONS FOR RENDERING SURFACES RECEPTIVE TO THE DEPOSITION OF AN ADHERENT ELECTROLESS METAL COMPRISE WATER AND A METAL COMPLEX CONSISTING OF (A) A PRECIOUS METAL SELECTED FROM A GROUP CONSISTING OF THE PRECIOUS METALS OF THE FIFTH AND SIXTH PREROIDS OF GROUPS VII AND I-B OF THE PERIODIC TABLE OF ELEMENTS, (B) A GROUP VI METAL OF THE PERIODIC TABLE OF ELEMENTS WHICH IS CAPABLE OF TWO VALENCE STATES AND (C) AN ANION CAPABLE FO FORMING A STABLE MOIETY WITH BOTH VALENCE STATES OF THE GROUP IV METAL, IN WHICH THE MOLAR RATIO OF (A) TO (B) TO (C) IS FROM ABOUT 1:1:3 TO 1:6.24. THE SOLUTIONS CAN BE PREPARED DIRECTLY FROM THE COMPONENTS OR, PREFERABLY BY DILUTING A CONCENTRATE. THE SOLUTIONS CAN BE STABILIZED AGAINST PRECIOU METAL SEPARATION BY ADDING AN ORGANIC MONO-OL, DIOL OR POLYOL, A FLUORINATED HYDROCARBON WETTING AGENT OR HYDROGEN FLYORIDE. PROCESS FOR RENDERING SURFACES RECEPTIVE TO THE DEPOSITION OF AN ELECTROLESS METAL ARE ALSO PROVIDED IN WHIC THERE ARE EMPLOYED THE NEW SENSITIZING SOLUTIONS.

3,682,571 Patented Aug. 8, 1972 Ser. No. 9,060

Int. Cl. C23c 3/02 U.S. Cl. 106-286 16 Claims ABSTRACT OF THE DISCLOSURE Optically clear sensitizing solutions for rendering surfaces receptive to the deposition of an adherent electroless metal comprise water and a metal complex consisting of (a) a precious metal selected from a group consisting of the precious metals of the fifth and sixth periods of Groups VIII and I-B of the Periodic Table of Elements, (b) a Group *IV metal of the Periodic Table of Elements which is capable of two valence states and (c) an anion capable of forming a stable moiety with both valence states of the Group IV metal, in which the molar ratio of (a) to (b) to (c) is from about 1:1:3 to 1:6:24. The solutions can be prepared directly from the components or, preferably, by diluting a concentrate. The solutions can be stabilized against precious metal separation by adding an organic mono-o1, diol or polyol, a fiuorinated hydrocarbon wetting agent or hydrogen fluoride.

Processes for rendering surfaces receptive to the deposition of an electroless metal are also provided in which there are employed the new sensitizing solutions.

CROSS-REFERENCES TO RELATED APPLICATIONS This application is a continuation-in-part of U.S. application Ser. No. 801,167, filed Feb. 20, 1969, which in turn is a continuation-in-part of U.S. application Ser, No. 712,575, filed Mar. 12, 1968, now abandoned, which in turn is a continuation of U.S. application Ser. No. 551,249, filed May 19, 1966, now abandoned, which in turn is a continuation of U.S. application Ser. No. 285,889, filed June 6, 1963, now abandoned, which in turn is a continuation of U.S. application Ser. No. 53,352, filed Sept. 1, 1960, now abandoned.

Generally stated, the subject matter of the present invention relates to new and useful precious metal sensitizing solutions. More particularly, the invention relates to solutions comprising a precious metal complex, and to concentrates thereof. The solutions are used to render surfaces of a substrate catalytic to the reception of an electroless metal.

BACKGROUND OF THE INVENTION The electroless deposition of a metal on either a metallic or non-metallic substrate usually requires pretreatment or sensitization of the substrate to render it catalytic to the reception of such deposit. Various methods have evolved over the years employing particular sensitizing compositions.

One of the earliest innovations employed a plurality of baths in which the substrate was subjected to a basic twostep process entailing immersion in a stannous chloride solution, followed by immersion in an acid palladium chlo ride solution. More recently, there has been proposed a unitary treating process which employs a colloidal dispersion, i.e., not a clear solution, of palladium and tin. See U.S. Pat. No. 3,011,920, Shipley.

The plural bath process, while readily effecting sensitization, deposits on metals a flash coating of the precious metal from the sensitizing bath. The precious metal so flash coated from the sensitizing bath disrupts the balance of the bath. Thus, to maintain catalytic activity, the addition of the precious metal bearing solution to the bath is required. This, of course, increases the cost of the plural bath process. Furthermore flash coating of precious metal normally leads to a low grade of adhesion, thereby materially affecting the bond and peel strength of an electroless metal deposit.

The unitary bath process on the other hand, While overcoming the problem of flash coating, has a diminished reactivity and requires longer periods of time for sensitization. Moreover, because colloids are involved, concentrates of such baths can not be prepared and shipped or stored because of destruction of the colloidal dispersion by flocculation. Concentrated unitary bath sensitizers are desirable because shipping and storage costs will be minimized; and process solutions are more simply put together (by dilution, for example).

Accordingly, it is a primary object of the present invention to provide a new sensitizer composition, and dilutable stable concentrates thereof, that are highly active, as well as new process to effect the sensitization of a substrate to render it catalytic to the reception of an electroless metal deposit.

Another object of the present invention is to provide new and useful stabilized compositions which are true solutions and methods for sensitizing substrates using them which substantially obviate the problem of non-adherent precious metal flash coatings.

It is an additional object of the invention to provide clear, stable sensitizing compositions and dilutable concentrates therefor and processes for the use thereof which materially reduce the time necessary to effect sensitization.

Yet another object of the invention is to provide dilutable, concentrated compositions and methods for sensitizing substrates with a precious metal which involves using very dilute treating solutions and thereby materially diminishing production costs.

Additional objects and advantages will be set forth in part in the description which follows, and in part will be obvious from the description, or may be realized by practice of the invention, the objects and advantages being realized and attained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims.

DESCRIPTION OF THE INVENTION To achieve the foregoing objects, and in accordance With its purposes as embodied and broadly described, the present invention relates to sensitizing solutions for rendering surfaces receptive to the deposition of an adherent electroless metal which comprise water and a metal complex consisting of three components:

(a) A precious metal selected from the group consisting of the precious metals of the fifth and sixth periods of Groups VIII and I-B of the Periodic Table of Elements;

(b) A Group IV metal of the Periodic Table of Elements which is capable of two valence states; and

(c) An anion capable of forming a stable moiety with both valence states of the Group IV metal, the molar ratio of components (a) to (b) to (c) being from about 1:1:3 to 1:6:24.

Also contemplated are stable, concentrated compositions, dilutable to optically-clear sensitizing solutions for rendering surfaces receptive to the deposition of an adherent electroless metal which comprise water and a metal complex consisting of three components: (a) a precious metal selected from the group consisting of the precious metals of the fifth and sixth periods of Groups VIII and I-B of the Periodic Table of Elements; (b) a Group IV metal of the Periodic Table of Elements, which is capable of two valence states; and (c) an anion capable of forming a stable moiety with both valence states of the Group IV metal, in which the molar ratio of precious metal to Group IV metal to anion is from about 1:1:3 to 1:624, and wherein the concentration of component (a) is at least about 1.5 grams/liter.

A preferred feature of the invention is a stabilized solution as above defined which includes a stabilizing amount of an organic mono-o1, diol or polyol, a fiuorinated hydrocarbon wetting agent or hydrogen fluoride. This embodiment is: a stabilized solution for rendering a surface receptive to the deposition of an adherent electroless metal which comprises water; a metal complex consisting of three components (a), (b) and (c) as defined above, the molar ratios of components (a) to (b) to being from about 1:1:3 to 1:6:24; and, as a stabilizing ingredient, a compound selected from the group consisting of organic mono-01s, diols and polyols, i.e., monohydroxy, dihydroxy and polyhydroxy organic compounds, a fiuorinated hydrocarbon wetting agent or hydrogen fluoride, the amount of said compound in said solution being at least sufficient to prevent separation therefrom of component (a) as a metallic film or precipitate.

Also contemplated are optically-clear sensitizing solutions which have been prepared by diluting the concentrates with water, an acid or a mixture of water and an acid, until the concentration of component (a) has been reduced to a concentration of from about 0.0003 grams/ liter to about 1.5 grams/liter.

Another feature of the invention is a process to prepare the concentrate comprising heating an aqueous mixture of the components (a), (b) and (c) at a temperature of from about 80 C. to about the boiling point until formation of the complex is complete.

Still another feature is a process to prepare the concentrate which comprises mixing an aqueous solution of components (a) and (c) with an aqueous solution of components (b) and (c) and heating the mixture at from 80 C. to its boiling point.

The present invention further relates to an improved process for rendering surfaces receptive to the deposition of an adherent electroless metal, as well as a process for electrolessly depositing a metal on a substrate which has been sensitized with the sensitizing solutions of the present invention.

An additional embodiment of the invention relates to particular acidic precious metal solutions for rendering surfaces receptive to the deposition of electroless copper.

The invention consists of the novel methods, processes, steps and improvements shown and described.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention. For example, while copper deposition is described, the teachings are applicable to nickel, palladium, cobalt, silver and gold deposition as well.

Accordingly, the present invention provides a significant improvement over the known sensitization techniques and compositions in that a procedure and composition is provided which will cause conductive and non-conductive materials to be so sensitized simultaneously that eflicient and uniform deposition of an adhering electroless metal thereon may be readily effective; thus permitting, by way of illustration, copper plating of the non-metallic surfaces of the side walls of the apertures defined in a plastic base material, as well as the concomitant adherent electroless deposition of copper on pre-formed copper surfaces on the plastic base. Illustrative of the conductive and nonconductive, metallic and non-metallic surfaces which can be plated uniformly with adherent electroless metal by use of the sensitizing solutions and techniques of the present invention are plastic surfaces and surfaces of metallic copper, iron-nickel, cobalt, silver, gold and alloys thereof, such as stainless steel, brass, sterling silver and the like. The present invention can be employed to render a surface catalytic to the reception of such metals as copper, nickel, palladium, cobalt, silver, gold, and the like.

A further advantage of the present invention is that sensitization as taught herein leads to practically complete avoidance of the deposition of a flash coating of precious metal from the sensitizing bath. A flash coating of precious metal normally leads to a low grade of adhesion. Also, avoidance of such deposition economically conserves the precious metal.

Yet another advantage of the present invention is realized in manufacturing presensitized plastic base members. In essence, the sensitizing solution can be directly incorporated in a plastic base at the time of its manufacture. This can be achieved by utilizing an organic sensitizing solution employing a solvent such as cyclohexanone, or a highly viscous paste like composition. Alternatively, the solution can be reduced to a solid form by employing a counter ion and directly incorporating such solid in a plastic base at the time of manufacture. Therefore, the advent of the prsent invention provides for the preparation of presensitized plastic solutions.

The most notable advantage occasioned by the advent of the present invention resides in the fact that such solutions possess a sensitizing activity at dilute levels. In essence, a complex is formed between the metals and the anion. The preferred palladium-tin-chloride complex of this invention, for example, in addition to being acid soluble, is soluble in organic solvents, e.g., dimethylformamide, cyclohexanone, hexane, dimethyl sulfoxide, and the like. It is optically clear in dilute solution. It is filterable through ultra-membranes through which colloidal sensitizers will not pass. It has an ultraviolet absorption peak at 395 m Once formed, concentrates of the complex according to this invention, may be diluted to levels of concentration which cannot easily and reproducibly be directly formulated in active sensitizing solutions. In other words the invention makes available stable sensitizing solutions comprising significantly lower concentrations of precious metal than can be produced by other known methods.

The treating or sensitization procedure which is an embodiment of this invention is an intermediate step between pretreatment or cleaning of the surfaces upon which the metal is to be electrolessly deposited and the actual deposition of the metal. The treatment to be afforded the surface to be plated depends upon the cleanliness of the material to be treated and associated factors. Thus, where the surface to be plated is either unclean or its cleanliness uncertain, the first step in the procedure for effecting deposition of adherent electroless metal is to clean thoroughly the article or panel upon which plating is to occur. This is desirably accomplished by scrubbing the panel with pumice or the like to remove heavy soils; rinsing with water; and subsequent removal of soiling due to organic substances from the panel and apertures defined therein with a suitable alkali cleaning composition. A typical alkaline cleaner composition is as follows:

Grams/liter Sodium isopropyl naphthalene sulfonate 3 Sodium sulfate 1 Sodium tripolyphosphate 14 Sodium metasilicate 5 Tetrasodium pyrophosphate 27 This operation is desirably performed at a temperature of to F. The surfaces to be plated are permitted to remain in the bath for a period of 5 to 30 minutes. Other suitable alkali cleaning compositions, such as conventional soaps and detergents, may also be used. Care should be used in selecting the detergent to insure that the specimen to be treated is not attacked by the cleaner.

Oxides are removed from copper panel surfaces and apertures by application of a light etching solution such as a 25 percent solution of ammonium persulfate in water as is described in Bulletin No. 86 of the Becco Chemical Division of the Food Machinery and Chemical Corporation, Buifalo, NY. The surface oxides also may be removed by application of the cupric chloride etchant solution described by Black in US. Pat. No. 2,908,557. This treatment should not exceed 2 to 3 minutes.

The treatment period and temperature are significant, particularly where the panel surfaces are formed of a conductive metal, in that elevated temperatures and extended periods of time beyond those described may result in removal not only of the oxide materials but of the conductive metal, such as copper foil, forming the surfaces of the panel. The panel is rinsed thoroughly after this step with water to remove all semblance of etching compounds. Care should be taken to avoid the formation of further oxide film during rinsing or as a result of air oxidation. Subsequent to rinsing, the panel may be inserted in a hydrochloric acid solution comprising 42 fluid ounces of hydrochloric acid per gallon of water for a period of from 2 to 5 minutes, and from this bath the panel is placed in the sensitization or treating solution of the present invention.

If the shape of the material permits, a sanding operation with a fine abrasive can also be used to remove oxides.

The sensitizing solution comprises an aqueous solution of a metal complex as defined above consisting of (a) a precious metal, (b) a Group IV metal and (c) an anion. It is optically-clear, i.e., non-colloidal. Among the precious metals that may be mentioned are palladium, platinum, gold, rhodium, osmium, iridium and mixtures of these metals. The inorganic and organic acid salts of these metals, and of the Group IV metals, such as the chlorides, bromides, fluorides, fluoroborates, iodides, nitrates, sulfates and acetates of stannous tin, titanium and germanium among others may be used. Other acid salts of the precious metals and Group IV metals will readily suggest themselves to those skilled in the art. The salts are preferably soluble in water, or in organic or inorganic acid aqueous solutions. Among the salts, the chloride is preferred, both for the precious metal, and the Group IV metal compound. The preferred precious metals and Group IV metal are palladium or platinum, particularly palladium and stannous tin. The concentration of the precious metal ion in the concentrates will be at least about 1.5 grams/liter. The upper limit will be about 100 grams/liter. In the sensitizing solution bath the precious metal concentration should be from at least about 0.0003 to about 1.5 grams/ liter of solution. While the higher end of this range causes sensitization to be completed in very abbreviated periods of time, e.g., ten seconds, the lower end of this range is normally more economic. The bath can be prepared directly or by diluting a concentrate.

In one manner of proceeding, the concentrates as defined above are diluted with Water or an aqueous solution of suitable acid. The concentrates are prepared by heating the salts of the precious metals and a Group IV metal salt in an aqueous solution of suitable acid as will be described hereinafter. Among the acids that may be mentioned are hydrochloric acid, hydrofluoric acid, fluoboric acid, hydroiodic acid, sulfuric acid and acetic acid. Preferably, the anion of the inorganic acid corresponds to the anion of the salt of the precious metals, or to the anion of the Group IV metal salt. Where the anions of the precious metal salt and the Group IV metal salt are the same, the anion of the acid should preferably correspond to the common anion of the salts. Where the anion of the precious metal differs from that of the Group IV metal salt, the anion of the acid preferably corresponds to the anion of the precious metal salt. However, acids having anions which differ from the anions of the precious metal salts or of the Group IV metal salt may also be used. Preferred anions are Cland SnCl The concentration of the acid in the sensitizing solutions and in the concentrates depends upon the strength of the acid employed. The concentration of the acid in the final solution should be at least 0.001 normal. At the upper end, especially in the concentrate, the concentration of acid may be as high as 15 normal, or even higher. When strong acids are used, the concentration of the acid in the sensitizing solution generally varies between about 0.02 and 7.5 normal. When weak acids are used, the concentration of the acid in the sensitizing solution approaches the upper limit given hereinabove. The concentration of acid in the sensitizing solution should, of course, be high enough to solubilize the salts of the precious metals and the Group IV metal and also high enough to render the solution suitable for use as a sensitizer for the material being treated. Care should be used in selecting the acid concentration to insure that the specimen being treated is not adversely attacked or corroded by the treating solution.

The Group IV metal ion concentration may vary widely but must be maintained in excess of a stoichiometric amount based on the amount of precious metal ions present in the sensitizing solution. Normally a large excess of, for example, stannous chloride, is maintained to allow for air oxidation of the stannous ion. Illustratively, concentrations of as high as 50 grams per liter of stannous chloride are not detrimental to the effectiveness of the sensitizing activity of the diluted solutions.

The sensitizing solutions and concentrates thereof, of the present invention may contain additional agents to stabilize the solution, e.g., against the effects of atmospheric oxidation. Such agents will include the use of additional quantities of the Group IV metal, as for example, additional stannous chloride may be added to a palladium, stannous chloride sensitizing solution, as well as compounds, such as organic ols, diols and polyols, the fluorinated hydrocarbon wetting agents and hydrogen fluoride. One result of the addition of such compounds to the sensitizing solution is the prevention of oxidation of the Group IV metals and the subsequent reduction of the precious metals.

As has been mentioned above, addition of the stabilizing ingredients to the sensitizing solutions of this invention improves the stability of the solutions and avoids formation of precious metal residues on surfaces, e.g., metal surfaces, exposed to such solutions. The nature of the stabilizing ingredients is not particularly critical. It should be soluble at least in an amount great enough to provide the desired stabilization effect. Evidence of effective stabilization is easily observed in comparison with control baths, i.e., those without the stabilizing ingredient present. These latter baths over a period of one week or so will be seen to deposit a metallic film of precious metals or to deposit a precipitate of precious metal. On the other hand, baths to which an effective amount of the stabilizing ingredient has been added will remain clear and be stable and storable for periods of at least two weeks and even longer. In any event, the minimum amount will vary somewhat from compound to compound but is easy to determine routinely. There is no apparent reason to limit the quantity of the stabilizing ingredient to the mini mum effective amount and often substantially more will be used, the choice being primarily dictated by economic considerations.

The stabilizing ingredient need not be completely water soluble, although for ease of formulation a high degree of water solubility is desirable. As will be obvious to those skilled in the art, the hydroxy compounds which can be used most efiiciently would be of relatively low molecular weight, polysubstituted with hydroxy groups or containing at least one hydroxy group and other functional groups such as sulfonic acid groups, phosphate groups, amino groups, halogens, carboxyl groups and the like, which tend to enhance solubility in polar media.

Similar considerations are involved in selecting the most efficient fluorinated hydrocarbon wetting agents. These will be characterized by a stable fluorocarbon tail and a solubilizing group, Z. The solubilizing group can be organic or inorganic, anionic, cationic, nonionic, amphoteric and water soluble. The wetting agents should be active in acidic solutions and have good thermal stability. Especially preferred are those wherein Z is anionic. A preferred species has the following spectrum of solubilities in acid media, in grams/ 1000 grams of solution: 12 /2% hydrochloric acid, 1; 37% hydrochloric acid, 0.1; 12 /z% nitric acid, 1; 70% nitric acid, 5; 12 /2% phosphoric acid, 1; 85% phosphoric acid, 1; 12 /z% sulfuric acid, and 97% sulfuric acid, 0.5. Another preferred species has the following solubility spectrum: 12 /2% hydrochloric acid, 10; 12 /2% nitric acid, 20; and 12 /2% sulfuric acid, 10. Particularly useful fluorinated hydrocarbon wetting agents of this type are sold under the trade name Fluorad by the Minnesota Mining and Manufacturing Company, St. Paul, Minn. The preferred species, described above, are designated Fluorad FC-95 and WG-98, respectively.

Preferred hydroxyl-containing stabilizing ingredients are open chain aliphatic mono-01s, diols or polyols of from about 1 to about 12 carbon atoms, e.g., methanol, ethanol, i-propanol, ethylene glycol, propylene glycol, 1,4-butane diol, glycerol, glucose, sucrose, and the like; and closed chain aliphatic or aromatic monoor di-carbocyclic mono-01s, diols or polyols of from about 6 to about 12 carbon atoms, e.g., cyclohexanol, cyclohexanediol, inositol, phenol, fl-naphthol, resorcinol, catechol, hydroquinone, pyrogallol, phloroglucinol, naphthresorcinol, and the like. It is preferred that these stabilizing ingredients have a water solubility of at least greater than about 4% by weight, especially those in the aromatic family. Particularly preferred stabilizing ingredients of this type are isopropanol, ethylene glycol, glycerol, resorcinol, catechol, hydroquinone, pyrogallol and phloroglucinol. These can generally be employed at an appropriate concentration in the range of from about 2 to about 50% by weight in the final mixture. The aromatic compounds will be employed at 2 to 7 wt. percent and the aliphatic compounds will be used in concentrations of from about 4 to 50 wt. percent, although higher and lower amounts can be formulated, as mentioned above. An especially preferred stabilizer is resorcinol at 20 to 70 grams/liter.

Illustrative fluorinated hydrocarbon Wetting agents have a formula selected from the group consisting of C 12,, CO-Z and wherein n is an integer of from 3 through 10, and Z is a hydrophilic group. Typically such surfactants are composed of molecules containing a perfluorinated tail portion and a hydrophilic head portion. The fluorocarbon portion of the molecules advantageously contains either 3 or 7 carbon atoms where the surfactant is a perfluorocarboxylic acid derivative, and 8 or 10 carbon atoms where the surfactant is a perfluorosulfonic acid derivative. Thus certain such surfactants have the formula C F COZ C F COZ C8F17SOZZ The hydrophilic portion of the fluorocarbon surfactants can be any typical hydrophilic group such as hydroxyl, alkali metal or alkaline earth metal substituted hydroxyl, an alkali metal group, an ammonium group,

amine groups, substituted amine groups, quaternary arnmonium salts, amide and substituted amide groups, and

the like. In one useful series, the hydrophilic tail portion of the radical can be a nitrogen-containing group having an amino or amido nitrogen or both. These can generally be employed at a concentration of from about 0.010 to 5.0 grams per liter in the final mixture, although higher and lower amounts can be formulated. Preferred results are obtained with from 0.025 to 1.0 g./liter and an especially good balance of stability and economy is obtained with 0.05 g./liter of the fluorinated surfactants. An illustrative surfactant for this embodiment of the invention has the formula: C F SO H. Useful too is hydrogen fluoride itself. This will be employed at 1 to 10 g./liter of solution and preferably at about 3 g./liter. Conveniently, this latter concentration is made by adding enough 60% aqueous HF to provide 50 ml./ liter.

While it should not be construed as limiting the invention, it is believed that the complexing reaction which occurs between the metals and the anion results in the formation of more than one and possibly several complexes. Empirically, these can be depicted as including complex anions of the formula mixtures thereof, wherein Me is Ru, Rh, Pd, Os, Ir, Pt, Au or a mixture thereof. Preferred complexes are those wherein Me is Pd or Pt.

The concentrations of the components of the complex are expressed in terms of molar ratio. Therefore, the molar ratio of precious metal to Group IV metal to anion of the complexes of the present invention is from about 1:1:3 to 1:6:24.

It is a preferred feature of this invention when preparing particularly preferred sensitizing solutions, most notably those containing complexes of palladium, stannous chloride, to employ an excess of Group IV metal and an excess of the anion. Especially preferred is to use the same anion as component (c).

It is important, when preparing the sensitizing solutions of the instant invention, that the aqueous solutions of components be added to each other and mixed so that the components of the aqueous solutions do not react to form a collodial dispersion or colloidal agglomerates. For example, if one of the aqueous solutions is added, slowly, to the other solution with vigorous agitation, which is standard procedure for preparing a colloidal dispersion, the components of the solutions will react and the mixed, reacted solutions, will form a colloidal dispersion of palladium with a portion of the colloidal palladium precipitated and agglomerated. Such colloidal dispersion is not the clear sensitizing solution of the present invention nor does such colloidal dispersion result in the improved sensitizing solution of the present invention nor the improved electroless plating resulting from such clear sensitizing solution.

One method found acceptable for producing a clear solution, when the aqueous solutions of components are mixed, is to dissolve the palladium chloride in a so1ution of hydrochloric acid and water and to quickly dissolve the stannous chloride in the palladium chloride. When so mixed, the stannous chloride acts as a reducing agent and the high concentration of stannous tin forms a complex with the palladium chloride and prevents the reduction of palladium chloride to metallic palladium. The palladium chloride and stannous chloride may be separately dissolved in equal portions of water, hydrochloric acid solution and then mixed together. Such solution must be aged for at least one hour at 25 C. before use. While aging, the color of the solution mixture will change from green to dark brown, indicating that the proper stannous chloride, palladium chloride acid salts have formed and that the solution is ready for use.

sensitizing solutions of the instant invention can be prepared as concentrates which may be stored, and shipped and diluted when the sensitizing solution is to be used for electroless plating. When properly prepared, in accordance with the teachings hereof, such diluted concentrates form true, clear solutions.

The concentrates of this invention can be prepared in oneor two-steps:

In the one-step preparation, an aqueous mixture, which contains components (a), (b) and (c), as defined above, in which mixture component (a) is present in a concentration of at least about 2.5 grams/liter, the molar ratios of (a) to (b) to (c) each being, respectively, 1: at least 1: at least 3; is heated at a temperature of from about 80 C. to about the boiling point of the mixture until formation of the metal complex is substantially complete.

In the two-step preparation, an aqueous solution containing a salt of component (a) in a concentration of from about 2.5 grams per liter up to about the limit of solubility of the salt in water at the boiling point and component (c); and an aqueous solution of components (b) and (c), the ratios of (a):(b):(c) each being respectively, 1: at least about 1: at least about 3, are first prepared. The two solutions are mixed together and heated at a temperature of from about 80 C. to about the boiling point of the mixture until formation of the complex is substantially complete. With palladium, 4.8 to 100 grams/liter (calculated as metal) can be conveniently used. Depending on the temperature, it is preferred to heat the mixture for from about to 90 minutes, although this is not critical.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A stable, concentrated composition, dilutable to an optically clear, sensitizing solution is prepared.

A first solution is prepared comprising the following ingredients:

Palladium chloride (g.) 62.9 Hydrochloric acid (37%, ml.) 200 Water to make total (ml.) 500 A second solution is prepared comprising the following ingredients:

Stannous chloride dihydrate grams 800 Hydrochloric acid (37%) ml 500 The first solution is added to the second with agitation and the mixture is heated to 85 C. and then boiled in a Well ventilated place for one hour.

The mixture is allowed to cool and there is obtained a concentrate according to this invention, which contains palladium in a concentration of about grams/ liter.

Example 2 Stable, concentrated compositions, dilutable to opticallyclear sensitizing solutions are prepared, according to Example 1.

The first solutions comprise, respectively:

Palladium chloride (g.) 3.10 93. 25 125 Hydrochloric acid (37%) (ml.) 200 200 200 Water to make (ml.) 500 500 500 The second solution is the same as that in Example 1. There are obtained concentrates according to this invention, which contain paladium in concentrations, respectively, of about 1.5, 45 and 60 grams/liter.

Example 3 A stable, concentrated composition, dilutable to an optically-clear sensitizing solution is prepared in one step. The following formulation is used:

Palladium chloride (g.) 62.5 Hydrochloric arid (37%) (ml.) 700 Stannous chloride dihydrate (g.) 800 Water to make total (ml.) 1295 Stannous chloride is dissolved in the hydrochloric acid, then the water is added. The palladium chloride is added with agitation. The mixture is heated to C. for 20 minutes, then boiled for 1.5 hours and cooled.

There is obtained a concentrate according to the invention which contains about 30 grams/ liter of palladium.

Example 4 A first solution is prepared comprising the following ingredients:

Palladium chloride (g.) 10 Hydrochloric acid (37%, ml.) 200 Water to make total .(ml.) 500 The palladium salt dissolves slowly in the acid-water mixture.

A second solution is prepared comprising the following ingredients:

Stannous chloride dihydrate (g.) 800 Hydrochloric acid (37%, ml.) 500 The first solution is added to the second with agitation, then the mitxure is boiled for 1.5 hours, during which time it changes color from blue or purple, through green, then straw-yellow and, finally, brown.

The mixture is allowed to col and there is obtained a concentrate according to this invention which contains palladium in a concentration of about 4.8 grams/liter.

Example 5 The procedure of Example 1 is repeated, respectively substituting for the palladium chloride, stoichiometricallyequivalent amounts of the following: ruthenium chloride (RuCl rhodium chloride (RhCl osmium chloride (OsCl iridium chloride (IrCl and platinum chloride .(PtCl Concentrates according to this invention containing, respectively, about 30 grams/liter of ruthenium, rhodium, osmium, iridium and platinum are obtained.

Example 6 A sensitizing solution according to this invention is prepared by taking 20 ml. of the concentrate from Example 1, diluting it with a mixture comprising 490 ml. of 37 percent hydrochloric acid and 490 ml. of water. The resulting solution will contain about 0.6 grams/liter of palladium in the form of the paladium-Stannous chloride complex of this invention and will have good catalytic activity.

In a similar manner, catalytically active sensitizing solu tions according to this invention are prepared by taking 0.01 ml. and 50 ml. portions of the concentrate of Example 1 and diluting them, respectively, to 1 liter in 1:1 by volume of a mixture of 37% aqueous hydrochloric acid and water. The solutions contain, respectively, 0.0003 grams and 1.5 grams of paladium per liter.

Example 7 The concentrates of Example 5 are diluted with enough 1:1 mixture by volume of 37% hydrochloric acid and water to provide sensitizing solutions according to this invention containing, respectively, 0.0006, 0.6 and 1.5 grams/liter of ruthenium, rhodium, osmium, iridium and platinum.

1 1 Example 8 A solution comprising the following ingredients is prepared:

Stannous chloride grams" 8.4 Hydrochloric acid (37%) ml 70 Palladium chloride grams... 4.0 Water to 1000 ml.

The palladium chloride is an aliquot of a solution containing PdCl and 37% hydrochloric acid, 50 grams and 50 ml./ liter, respectively.

The solution is allowed to stand for approximately one hour at room temperature, during which time the color will change, starting with blue-black, then dark green and deep brown and finally a dark brown. At this stage it is believed that the solution contains considerable quantities of the catalytically active palladium, stannous chloride complex and requires the addition of an excess of stannous chloride. It is preferred to add 50 grams, although the amount is nominal and more-01 even less-of an excess will do. The solution is then diluted with a stannous chloride, hydrochloric acid solution to give a composition having the following concentrations of components:

Palladium chloride gram/liter 1 Stannous chloride grams/liter 60 Hydrochloric acid (37%) ml./liter 100 The solution is catalytically active and will remain stable so long as the stannous tin and hydrochloric acid are maintained.

If the additional stannous chloride is not added to the original solution when it achieves a dark brown color, the solution will be unstable and a black precipitate will form after standing for several days. The black precipitate is believed to contain a palladium, stannous complex and palladium metal. The precipitate complex can be redissolved in concentrated hydrochloric acid and will be catalytically active. To achieve stability excess stannous tin should be added to the solution.

The precipitated palladium metal will not redissolve in the concentrated hydrochloric acid unless an oxidizing agent such as hydrogen peroxide is added.

A catalytically active solution containing 1 gram/liter of palladium chloride is prepared as described above, but as a stabilizing ingredient, there is added 50 grams/liter of resorcinol. Instead of resorcinol, the solution can be stabilized with ethylene glycol, 700 ml./liter, or isopropanol, 450 ml. per liter.

Example 9 An additional dilute solution is prepared in substantially the same manner as hereinabove described, having both a high concentration of the preferred complex, as well as good catalytic activity. The dilute solution consists of the following concentrations of components:

Palladium chloride gram/liter 0.05 Stannous chloride "grams/liter" 5.00 Hydrochloric acid (37%) ml./liter 350 Attempts to duplicate a catalytically active solution comprising the above concentrations of components directly, i.e., without diluting a concentrate, will not be successful.

The inactive solution is prepared by dissolving 1 gram of palladium chloride in water containing 200 ml. of 37% hydrochloric acid and then dissolving 80 grams of stannous chloride in the solution. The resulting solution is not catalytically active even though a palladium, stannous chloride complex is formed as will be evidenced by the fact that the original palladium chloride, hydrochloric acid solution looses its original properties. In essence, the palladium chloride, hydrochloric acid solution will have a light yellow color and will galvanically deposit palladium metal on a strip of copper metal. The addition of stannous chloride will change the color of the solution to deep green and the solution will no longer galvanically deposit palladium metal on a strip of copper.

Example 10 A solution is prepared comprising the following ingredients:

Gold chloride solution 2% ml 5 Stannous chloride grams 17 Hydrochloric acid (37%) ml 50 Water, to 1000 ml.

Dissolve the stannous chloride in water and add the hydrochloric acid. To this solution add the gold chloride solution with agitation. A small amount of precipitate forms, which is removed, and the remaining solution comprises the gold, stannous chloride complex. The solution is catalytically active.

Example 11 A sensitizing solution comprising the following ingreclients is prepared:

Palladium chloride (PdCI gram 0.25-1 Hydrochloric acid (37%) ml 40 60 Stannous chloride (SnCl -2H O) grams 12-22 Water, to 1000 ml.

This composition is formulated by dissolving palladium chloride in water containing 40 to 60 ml. of 37% or 1 N hydrochloric acid. Dissolution is slow and continues normally for several hours at room temperature. When the palladium is completely dissolved the stannous chloride is dissolved in the resulting solution. It is noted in this regard that when the stannous chloride is first dissolved in the aforesaid solution a green color may be noticed initially. After about one hour, however, the solu tion will change to a dark brown color, which coloration indicates that the solution, which is made up to 1 liter with water, is ready for use. The panel being prepared for plating is then immersed in the sensitizing solution for a period of from 5 to 20 minutes at room temperature.

It is noted that while the use of stannous chloride is preferred in the practice of the present invention other stannous tin compounds such as, for example, stannous fluoborate and stannous sulfate are also suitable for use in the seeding, i.e., sensitizing compositions described herein and in equivalent concentrations.

Example 12 The following is an example of a suitable sensitization solution having high acid content:

Palladium chloride grams 4 Stannous chloride do 25 Hydrochloric acid (37%) 1 liter.

Example 13 The following is an exampe of stannous fluoborate compositions which may be used as the sensitizing solution:

Palladium chloride gram 1 Hydrochloric acid (37%) ml 50 Stannous fiuoroborate (Sn(BF 47% solution) Water, to 1000 ml.

Example 14 The following is an exampe of stannous sulfate compositions which may be used as the sensitizing solution:

Palladium chloride grams 4 Hydrochloric acid (37%) ml Stannous sulfate grams 20 Water, to 1000 ml.

The stannous fiuoroborate and stannous sulfate sesitizing solutions are made up in a manner similar to that described hereinabove in connection with the stannous chloride solutions.

The following examples further illustrate typical sensitizing solutions of the present invention.

Example 15 Platinum chloride gram 1 Stannous chloride grams 20 Hydrochloric acid (37%) ml 40 Water, to 1000 ml.

Example 16 Rhodium chloride grams 4 Stannous chloride grams 20 Hydrochloric acid (37%) ml 40 Water, to 1000 ml.

Example 17 Palladium chloride gram 1 Acetic acid (99.5%) ml 880 Stannous chloride grams 20 Water, to 1000 ml.

The following examples illustrate sensitizing solutions of the present invention which have been stabilized with fluorocarbon wetting agents and hydrofluoric acid, respectively.

With respect to the sensitizing process aspect of this invention, after being immersed in the diluted solution containing from 0.0005 to 2.5 grams/liter of precious metal for the suitable period of time, the panel surfaces including any side walls of the apertures defined therein are thereafter thoroughly rinsed with water to entirely remove the sensitizing solution therefrom. The panel is then passed through a further bath of hydrochloric acid, suitably diluted, e.g., about 10 percent, the passage employing a period of from 10 to 20 seconds, and the panel is again rinsed with water prior to immersion in a suitable electroless plating bath.

Conventional electroless plating baths may be used for the deposition of the adherent metal after sensitizing of the plating surface with the compositions of the present invention. The electroless copper deposition may be followed by electroplating with copper or other metals to build up copper thicknesses of 0.01 to 0.002 inch or greater.

As an example, in the printed circuit industry, electroless copper is ordinarily deposited on apertures formed in plastic insulation sheets which have conductive copper foil laminated on both top and bottom surfaces. Following deposition of electroless copper, the circuits are conventionally electroplated with copper or other metals to build up copper thicknesses of 0.001 to 0.002 inch or greater.

Electroplated copper is required over the electroless copper to form rugged conductive copper on the walls of the aperture approximately 0.001 inch thick or greater.

However, in the prior art the adhesion between the electroless copper and the foil originally laminated to the plastic sheet has been very poor due to a poorly adherent flash coating of precious metal from the catalyzing step. The subsequent electrodeposits fail because of the flash coating and could easily be stripped off merely by the application of pressure sensitive adhesive coated cellophane tape such as Scotch cellophane tape manufactured by the Minnesota Mining and Manufacturing Company. Therefore, prior to the advent of the present invention, in order to achieve adherent coating, the surface of the copper foil had to be mechanically abraded before electroplating to remove all trace of the electroless copper deposits. This was a costly and time-consuming operation. By using the sensitizing solutions of the present invention, however, it is not necessary to abrade the surface to remove the electroless copper. The electroplated copper may be deposited directly and will adhere so strongly that if the plastic base sheet is broken, the copper foil may be bent back and forth on itself until it breaks but no separation is evident between the original laminated foil and the electroplated copper film.

Although the invention has been described and illustrated by reference to particular embodiments thereof, it will be understood that in its broadest aspects the invention is not limited to such embodiments, and that varia tions and substitution of such equivalents may be resorted to within the scope of the appended claims.

I claim:

1. A process for the preparation of a concentrated composition dilutable to an optically clear, sensitizer solution for rendering a surface receptive to the deposition of an adherent electroless metal which comprises water and a metal complex consisting essentially of:

(a) an unreduced ion of a precious metal selected from the group consisting of the precious metals of the fifth and sixth periods of Groups VIII and IB of the Periodic Table of Elements;

(b) a Group IV metal of the Periodic Table of Elements which is capable of two valence states; and

(c) an anion capable of forming a stable moiety with both valence states of the Group IV metal, the molar ratio of components (a) to (b) to (c) being from about 1:123 to 1:6:24, and wherein the concentration of component (a) is at least about 1.5 grams/ liter which comprises heating an aqueous mixture containing said components (a), (b) and (c), in which mixture component (a) is present in a concentration of at least about 1.5 grams/liter, at a temperature of from about C. to about the boiling point of said mixture, until the formation of said metal complex is substantially complete.

2. A process for the preparation of a concentrated composition as defined in claim 1 which comprises;

(1) providing an aqueous solution containing a watersoluble salt of said component (a) in a concentration of from about 1.5 grams/ liter up to about the limit of solubility of said salt in water and component (2) providing an aqueous solution of said components (b) and (c), the ratios of components (a):(b):(c) being 1: at least 1: at least 3; and

(3) mixing said solutions and heating the mixture at a temperature of from about 80 C. to about the boiling point of said mixture.

3. A stabilized sensitizer solution for rendering a surface receptive to the deposition of an adherent electroless metal consisting of:

(A) water;

(B) an effective, sensitizing amount of a metal complex consisting essentially of:

(a) an unreduced ion of a precious metal selected from the group consisting of the precious metals of the fifth and sixth periods of Groups VIII and I-B of the Periodic Table of Elements;

(b) a Group IV metal of the Periodic Table of Elements which is capable of two valence states; and

(c) an anion capable of forming a stable moiety with both valence states of the Group IV metal, the molar ratio of components (a) to (b) to (c) being from about 12123 to 1:6:24; and, as a stabilizing ingredient,

(C) a compound selected from the group consisting of a fluorinated hydrocarbon wetting agent and hydrogen fiuoride, the amount of said compound in said solution being at least sufficient to prevent separation therefrom of component (a) as a metallic film or precipitate.

4. A solution as defined in claim 3 wherein stabilizing ingredient C is selected from the group consisting of a wetting agent characterized by a stable fluorocarbon tail and a solubilizing group and hydrogen fluoride.

5. A solution as defined in claim 4 wherein said compound is a wetting agent wherein said solubilizing group is anionic and water soluble.

6. A solution as defined in claim 3 wherein component (a) is palladium.

7. A solution as defined in claim 6 wherein component (b) is tin.

8. A solution as delned in claim 7 wherein component is Cl, SnCl or a mixture thereof.

9. A solution as defined in claim 3 wherein component (a) is platinum.

10. A solution as defined in claim 9 wherein component (b) is tin.

11. A solution as defined in claim 10 wherein component (c) is Cl, SnCl or a mixture thereof.

12. A solution as defined in claim 11 wherein said metal complex includes an anion of the formula:

References Cited UNITED STATES PATENTS 3,011,920 12/1971 'Shipley, Jr 1l747 A 3,532,518 10/1970 DOttavio 1061 3,099,608 7/1963 Radovsky 1l747 A 3,563,784 2/1971 Innes et al l17-160 R 3,579,365 5/1971 Grunwald et al 1061 3,515,649 6/1970 Hepfer 117160 R 3,425,946 2/1969 Emons, Jr 106--1 3,533,923 10/1970 Moore, Jr. et al. 1061 OTHER REFERENCES Khattak et al., Tin (II) Chloride Complexes of Platinum Metals: the Palladium (II)Tin (II) System, In Chemical Communications, Nov. 17, 1965, p. 400.

Young et al., Complexes of Ruthenium, Rhodium, Iridium, and Platinum With Tin (II) Chloride. In Jour. Chem. Soc., 1964, No. 992, pp. 5176-5189.

ALFRED L. LEAVITT, Primary Examiner J. A. BELL, Assistant Examiner US. Cl. X.R.

106-1, 287; 11747 A, 47 R, E, R; 20430; 252-182 

